1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention also relates to a method for manufacturing the electrophotographic photoreceptor, and an electrophotographic image forming method and apparatus and a process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Background
Recently, development of the information processing systems using electrophotography is remarkable. In particular, improvement in the print qualities and reliability of laser printers and digital copiers, in which image information is recorded using light after information is converted to digital signals, is remarkable. In addition, the technique is applied to full color laser printers and full color digital copiers while being combined with a high speed recording technique. Therefore, a need exists for a photoreceptor which can produce high quality images while having a good durability.
As the electrophotographic photoreceptor (hereinafter referred to as a photoreceptor) for use in electrophotographic image forming apparatus, inorganic photoreceptors using a material such as selenium or amorphous silicon and organic photoreceptors are known. Among these photoreceptors, organic photoreceptors have been typically used now because of having low costs and good productivity and being non-polluting.
Specific examples of the organic photoreceptors include photoreceptors having the following photosensitive layers:    (1) photosensitive layers including a photoconductive resin typified by polyvinylcarbazole (PVK);    (2) photosensitive layers including a charge transfer complex typified by polyvinylcarbazole-2,4,7-trinitrofluorenon (PVK-TNF);    (3) photosensitive layers including a pigment dispersion typified by a phthalocyanine-binder system; and    (4) functionally-separated photosensitive layers using a combination of a charge generation material and a charge transport material.
Among these organic photoreceptors, the functionally-separated photoreceptors are widely used now because of having high photosensitivity, good durability and good flexibility in selecting appropriate charge generation materials and charge transport materials.
The mechanism of forming an electrostatic latent image on a functionally-separated photoreceptor is as follows:    (1) when imagewise light irradiates a charged photoreceptor, the imagewise light is absorbed by a charge generation material in a charge generation layer after passing through a transparent charge transport layer located overlying the charge generation layer;    (2) the charge generation material absorbing light generates a charge carrier;    (3) the charge carrier is injected into the charge transport layer and transported through the charge transport layer (or the photosensitive layer) due to an electric field generated by the charge formed on the surface of the photoreceptor; and    (4) the charge carrier neutralizes the charge on the surface of the photoreceptor, resulting in formation of an electrostatic latent image.
However, organic photoreceptors have drawbacks in that the photosensitive layers thereof are greatly abraded after repeated use, thereby deteriorating the potential formed on the photoreceptors and photosensitivity thereof, and in addition the resultant images have background development caused by scratches on the surface of the photoreceptor and low image density. Therefore, it has been attempted to improve the abrasion resistance of photoreceptors. In addition, recently electrophotographic image forming apparatus are required to be small in size and to perform high speed image formation. Therefore, the photoreceptors for use in such image forming apparatus need to have good durability.
In attempting to improve the durability of photoreceptors, the following methods have been proposed:    (1) a protective layer having good lubricity is formed as an outermost layer;    (2) a crosslinked protective layer is formed as the outermost layer; and    (3) a protective layer including a filler is formed as outermost layer.
Among these methods, the third method is effective at improving the durability of the photoreceptors. However, when a filler having a high insulation property is included in the protective layer, the resistance of the photoreceptor increases, and thereby a problem of serious increase in residual potential (hereinafter referred to as a residual potential increasing problem) is caused. This problem is considered to be caused by increase in resistance thereof and number of charge trap sites therein. In contrast, when an electroconductive filler is used in the protective layer, the resistance of the photoreceptor decreases, and therefore the problem of serious increase in residual potential is not caused. However, other problems such that outlines of the resultant images blur (hereinafter referred to as a blurred image problem), and thereby image qualities deteriorate.
Therefore, a technique in that a filler having a relatively low insulation property is used for the protective layer while the photoreceptor is heated by a heater such as drum heaters to prevent occurrence of the blurred image problem. In this case, the blurred image problem can be avoided, but the diameter of the photoreceptor has to be increased. Therefore, this technique cannot be applied to photoreceptors having a small diameter and small-sized image forming apparatus); In addition, by using a drum heater, the resultant image forming apparatus have the following drawbacks:    (1) the electric power consumption of the image forming apparatus increases; and    (2) the waiting time (i.e., the time needed for activating the image forming apparatus from switching on) is long.
In contrast, when a filler having a high insulation property is used, the residual potential increasing problem occurs. In this case, the potential of lighted portions of the photoreceptor increases, and thereby the image density and half-tone property of the resultant images deteriorate. In attempting to solve this problem, a technique in that the potential of non-lighted portions of the photoreceptor is increased is proposed. However, in this case, other problems such that background development is caused due to increase in electric field formed on the photoreceptor, and the life of the photoreceptor is shortened occur.
In addition, in attempting to solve the residual potential increasing problem, published examined Japanese Patent Applications Nos. 44-834, 43-16198 and 49-10258 have disclosed techniques in that a protective layer having a photoconductivity is formed as an outermost layer. However, the photoreceptors have a drawback in that the quantity of the imagewise light reaching the photosensitive layer located below the protective layer decreases because the protective layer absorbs the irradiated light, resulting in deterioration of the photosensitivity of the photoreceptor.
In attempting to improve the abrasion resistance, published unexamined Japanese Patent Application No. (hereinafter referred to as JOP) 57-30846 discloses a photoreceptor in which a protective layer including a filler such as a metal or a metal oxide, which has a particle diameter not greater than 0.3 μm, is formed as an outermost layer of the photoreceptor, to increase the transparency of the protective layer (i.e., to prevent increase of residual potential). By using this method, the increase of residual potential can be prevented to some extent. However, the effect is insufficient, and the residual potential increasing problem cannot be fully solved. This is because the problem is mainly caused by charge trapping by the added filler, which depends on the dispersion conditions of the filler in the protective layer. Even when the particle diameter of the filler is greater than 0.3 μm, the transparency of the resultant protective layer can be increased if the filler has good dispersibility. In contrast, even when the particle diameter is not greater than 0.3 μm, the transparency of the resultant protective layer is low if the filler has poor dispersibility.
Further, JOP 4-281461 discloses a photoreceptor, in which a charge transport material is included in a protective layer together with a filler to prevent increase of residual potential while maintaining good abrasion resistance. By including a charge transport material in the protective layer, the mobility of charges in the protective layer can be improved and thereby the increase of residual potential can be prevented to some extent. However, as mentioned above, the problem is mainly caused by increase in number of charge trapping sites, and therefore, there is a limitation on the residual potential improving effect by this method. Therefore, in this case, the thickness of the protective layer and the content of filler in the protective layer have to be decreased, and thereby good durability cannot be imparted to the resultant photoreceptor.
Further, in attempting to solve the residual potential increasing problem, JOP 53-133444 discloses a photoreceptor in which a Louis acid is included in the protective layer; JOP 55-157748 discloses a photoreceptor in which an organic proton acid is included in a protective layer together with a filler; JOP 2-4275 discloses a photoreceptor in which an electron accepting material is included in a protective layer; and JOP 2000-66434 discloses a photoreceptor in which a wax having an acid value not greater than 5 mgKOH/g is included in the protective layer.
By using these methods, the injection property of charges at the interface between the protective layer and the charge transport layer can be improved, because portions having a relatively low resistance are formed in the protective layer and thereby the charges can easily reach the surface of the protective layer. However, these photoreceptors have a drawback in that the blurred image problem is caused. In addition, when an organic acid is included in the protective layer, dispersibility of the filler deteriorates, and thereby good residual potential improving effect cannot be produced.
In order to produce high quality images using a photoreceptor having a filler-containing protective layer, not only occurrence of the residual potential increasing problem and the blurred image problem has to be prevented, but also the photoreceptor has to have a property such that charges generated in the photoreceptor can linearly reach the surface of the photoreceptor without being obstructed by the filler in the protective layer.
Whether or not charges can linearly advance through the protective layer depends on the dispersibility of the filler in the protective layer. Specifically, when the filler in the protective layer is aggregated, advancement of the charges, which are injected into the protective layer from the charge transport layer, to the surface of the protective layer is obstructed by the filler in the protective layer. As a result, toner particles are scattered in the resultant toner image, resulting in serious deterioration of the resolution of the toner image.
In addition, in a case where image writing light is scattered at the filler included in the protective layer and thereby the light transmission is deteriorated, the resolution of the resultant toner images is also deteriorated. The scattering of image writing light also changes depending on the dispersion conditions of the filler in the protective layer.
Further, the dispersion conditions of the filler in the protective layer greatly influence on the abrasion resistance of the protective layer. Specifically, a filler is unevenly dispersed in the protective layer, the abrasion resistance of the protective layer deteriorates.
Therefore, in the photoreceptor having a protective layer in which a filler is dispersed to improve the durability of the photoreceptor, it is important to improve the dispersibility of the filler therein.
However, there is no photoreceptor which includes a filler-containing protective layer and which can produce high quality images without causing the blurred image problem and the residual potential increasing problem while having good durability. As mentioned above, when a drum heater is used for preventing occurrence of the blurred image problem, other problems in that the image forming apparatus becomes large in size, and the power consumption thereof increases are caused.
JOP 2000-231204 discloses an aromatic compound having dialkylamino group which serves as an acid scavenger. It is described therein that this compound is effective at improving image qualities after repeated use, namely effective at solving the blurred image problem caused by oxidizing materials. However, the compound has poor charge transport ability, and therefore the compound cannot be used for photoreceptors for use in high speed image forming apparatus. Therefore, the compound cannot be included in a photoreceptor in a small quantity, and thereby the effect is little.
Further, JOP 60-196768 and Japanese Patent No. 2884353 (i.e., JOP 03-96961) have disclosed stilbene compounds having a dialkylamino group. It is reported by Itami et al. in Konica technical Report, Vol. 13, P 37, 2000 that the compounds are effective at improving the blurred image problem. However, the dialkylamino group of the stilbene compounds has a strong mesomeric effect (i.e., +M effect) against the triaryl amine structure which is a charge transport site. Therefore, the ionization potential of the entire system seriously decreases. Therefore the charge retaining ability of the charge transport material therein deteriorates at the beginning or after repeated use. Therefore the photoreceptor is not practically used. Even if the stilbene compounds are used together with other charge transport materials, the resultant photoreceptors have very low photosensitivity and high residual potential. This is because the ionization potential of the stilbene compounds is very low relative to that of the charge transport materials and therefore the stilbene compounds serve as trap sites.
Because of these reasons, a need exists for a photoreceptor which has good durability and which can produce high quality images without causing the blurred image problem and the residual potential increasing problem.